Method of operating electric furnaces



May 17 1927.

S. ARNOLD. 8D

METHOD OF dPERATING ELECTRIC FURNACES- Filed April 14, 1926 INVENTOR Elm W Patented May 17, 1927.

UNITED STATES SAMUEL ARNOLD, 3!), 01 BEN AVON, PENNSYLVANIA.

METHOD OF OPERATING ELECTRIC FURNACES.

Application filed April 14, 1926. Serial No. 101,925.

The present invention relates broadly to the 'art of melting and refining metals in electric furnaces, and more particularly to an improved voltage control for such furnaces voltage, the intermediate voltage being effective during such times as excessive surging has heretofore been encountered when attempting to use a high voltage In starting suitable for use during the melting and refining of steel, special alloys, and the like.

It was originally customary in the art of electric furnaces to utilize a single voltage for both the melting and the refining operal0 tions. Such a voltage was 'not satisfactory for either of these conditions, as it was 'nccessarily acompromise which was too low for melting and too high "for refining.

In order to overcome objections such as existed with a single voltage system, a double voltage was obtained by means either of taps on the transformer or by a delta Y connection. Such a double voltage system was inherently better in that it provided avoltage which could be high enough for effective melting and low enough for retining. 1 have found, however, that such a system has certain inherent objections in that in starting the melt with a high voltage. fluctuations of load are great enough to be extremely objectionable. The low voltage, on the other hand, could not be used to start the melt as it would not be great enough to establish contact. Furthermore, alter a charge has become nearly melted and the pool of molten metal is large in proportion to the unmelted scrap remaining, there is a very considerable heat radiation to the side walls when a high voltageis utilized. This as results in arapid deterioration of the re fractory lining of the furnace, whereby its effective length of life is materially decreased. 1f the low voltage in a system as described has been high enough to permit its use for starting the melt, it has been too high from the standpoint of good refractory life. and also too high for proper metallurgical conditions during refining.

It is one of the objects of the present invention to provide a flexible operating system for electric furnaces providing a multi ple voltage control whereby arcs of different characteristics may be obtained, and capable of providing at least three different voltages whereby the operator may choose a voltage at any time which is particularly suited to the conditions existing within the furnace.

The present invention contemplates more particularly an electric furnace operation embodying a high, low and intermediate the melt. or available for use when the pool of molten metal becomes large in proportion to the remaining unmelted scrap, the apparatus and circuit connections for securing such operation constituting the subject matter of a divisional application Serial No. 181,092, filed April 5, 1927. In the accompanying figure of the drawings, which is a diagrammatic illustration of one form of circuit by means of which the present invention may be carried out, the embodiment shown is for purposes of illustration only, and not for purposes of limiting the present invention, it. being understood that changes in the operating conditions providing the results herein contemplated might be obtained in various ways. in the drawings, there are illustrated the high voltage windings 1. 2 and 3 of a three phase transformer, or a bank of three single phase transformers. The low tension coils 4, 5 and 6 for cooperation therewith are preferably connected in, delta, as will be apparent from the drawings, with leads which may be permanently or otherwise suitably connected to electrodes 7, 8 and 9. These electrodes may be of any desired standard construction for operation in known manner in connection with an elec tric furnace 10. Cooperating with the high voltage wind mgs s a main line automatic overload protected oil circuit breaker 11 of any usual construction. Leading to the circuit breaker 11 are the power lines 12, 13 and 14:, which, when the circuit breaker is closed, will be operatively connected to the ends of the primary or high voltage transformer windings by the leads 12, 13 and 14'. respectively.

The present invention further contemplates the utilization of ta changing oil 1 circuit. breakers 15. 16 and 17. which are preferably electrically or mechanically interlocked. as well understood in the art, whereby only one of the breakers 15, 16 or 1'?" may be closed at any one time. Cooperating 1 with the breakers 15 and 16 are reactor coils 18, 19 and 2G. The three terminals 15, 15 and 15 of the breaker 15 are respectively connected by leads a, Z and c to points a, b and 0'" on the high voltage wind- 1 ings 1, 2 and 3, while the opposite termi'- nals 15, 15 and 15 of the breaker 15 are respectively connected by the wires d. c and f to the intermediate points rl, e and f of the reactor coils 18, 19 and 20.

In similar manner. the three terminals 16*, 16 and 16 of the breaker 16 are respectively connected by wires 7, h and 2' to intermediate points {,1', h and i of the high voltage windings 1. 2 and 3. The opposite terminals 16 16 and 16 of the breaker 16 are, in turn, respectively connected by the wires k and l to one of the ends of the reactor coils 18, 19 and 20, respectively. The opposite ends of these reactor coils are respectively connected in known manner, and as indicated in the drawings, to the points on the high voltage windings with which the wires 12, 13 and 1 1- respectively connect.

With a system of the character described, when it is desired to start the melt in the electric furnace, at which times an intermediate voltage is desired in order to reduce surging or load fluctuations to a minimum, the circuit breaker 11 is closed whereby the power supply lines 12. 13 and 14 are connected to the ends of the primary or high voltage transformer windings. The circuit breaker 15 is then closed, thereby connecting the contacts 15, 15 and 15, respectively, to the points d, e and f of the reactor coils.

Due to the connection. of these reactor coils to the high tension windings, itwill be apparent that a delta connection of the high voltage windings is formed with a portion of the reactance in series with the delta. It will be apparent that the taps on the primary transformer windings are so arranged that with the breaker 15 closed, the

' desired intermediate voltage for starting the melt is obtained.

After the charge has become partly melted and a comparatively small pool of molten metal is formed, it is then both feasible and desirable to utilize a higher voltage for quick melting. Such a higher voltage may be effectively ,utilized as the arc will be buried in the scrap to a large extent at least, and the radiations therefrom to the side walls will be reduced to a minimum. This high voltage may be obtained by opening the breaker 15 and closing the breaker 16. This will be effective for connecting the taps or points g, h and i on the primary winding to the respective ends of the reactor coils 18, 19 and 20.

The opposite ends of the reactor being permanently connected to the primary winding, as before described, a second delta con-- nection is established by the breaker 16 which utilizes only a portion of the primary transformer winding for each phase with a larger portion of the reactor connected in the delta. In this manner, the desired high voltage is obtained, it being understood that V the taps on the primary winding of the transformer, which may be especially constructed for furnace use, are so arranged that the proper high voltage value at this time will be obtained. This condition may be maintained until such time as the charge becomes nearly melted and the pool of molten metal large'in proportion to the remaining scrap whereby objectionable heat radiation to the refractory of the furnace would again exist. At this time, it is desirable to change to the intermediate voltage for finishing the melt, this change being effected by opening the breaker 16 and closing the breaker 15.

After the charge is completely melted, the voltage provided by the breaker 15, as well as that provided by the breaker 16, is too high for the refining operation, and it becomes necessary to change to a lower refining voltage. This may be accom lished by opening the breaker 15 and c osing the breaker 17, this breaker being eifective for connecting the points 21, 22 and 23 of the primary winding together, thereby forming a Y connection effective for the desired refining voltage.

The leads joining the taps on the transformer or high voltage windings, may obviously be so changed that a wide and varied selection of voltages is obtainable.

' Certain advantages of the present invention arise from the method of operating electric furnaces by means of an intermediate voltage at the beginning and at the completion of the melt.

Still other advantages arise from the operation of an electric furnacein such manner that at least three difl'erent voltages are obtainable during different periods of the melting or refining operations.

Further advantages of the invention arise from the operation of an electric furnace in such manner that melting of the charge is obtained under the most expeditious conditions with respect both to rapidity of melt ing and conservation of the furnace refractories.

Still further advantages arise from the operation of an electric furnace by means of an arc voltage which is lower at the beginning of the melt than during the main portion of the melt and higher than the arc voltage during refining.

Still further advantages result from the provision of a combination of a tapped transformer and delta Y connections.

Still further advantages arise from the provision of a multiple voltage control embodying a combination of a tapped transformer and delta Y connections with reactance which is variable for different delta connections.

a sence 3 an intermediate voltage for starting the melt, changing to a higher voltage until the melting is nearly completed. and then again changing to an intermediate voltage to at least complete the melt.

4, In the method of operating electric arc furnaces, thesteps comprising providing at least three available voltages. starting the melt, then utilizing a high voltage for the main melting period, and then changing to an intermediate voltage for at least the comple'tion of the melt.

5. In the method of operating electric arc furnaces, the step comprising initiating the melt at a voltage higher than the refining voltage and lower than the voltage utilized for the main melting period.

6. -ln-the method of operating electric arc furnaces, the step comprising completing a melt at a lower voltage than that used for the main melting period and as high as that used for startin the melt.

7. In the met od of operating electric arc furnaces, the step comprising utilizing an intermediate voltage for starting the melt, changing to a high voltage until the charge is nearly melted, again changing to at least an intermediate voltage to finish the melt, and the utilizing a low voltage for the relining period.

8. in the method of operating electric arc furnaces, the step comprising providing at least three available voltages, and rotecting the furnace walls against undue eat radiation by utilizing an intermediate voltage for starting and completing a melting operation. a higher voltage during such of the melting period as suflicient scra is available to bury the arc, and a still lbwer voltage for refining.

In testimony whereof I have hereunto set my hand.

SAMUEL ARNOLD, 3o.

CERTIFICATE l GQRREC'llON.

Pam: No. 1,629,196.

Granted Mayil, 1927, to

SAMUEL ARNllLlIl 3D.

It ie hereby certified that error appears in the printed specification of the above numbered patent requiring correction as ioilows: Page 3, line 41, claim 7, strike out the words "at least" and insert the same to follow after the word "to" in line 42; line 43, for the article "the" iirst occurrence, read "thenj'; and that the said Letters Patent should he read with these corrections therein that the same may conform to the record oi the case in thePatent Office.

Signed-and sealed this 14th day at tim A. a. 1927.

Seal

. In 'y Acting Commissioner of Patents." 

